Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV (extreme ultraviolet) range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
In the illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV range, in particular the use of facet mirrors in the form of field facet mirrors and pupil facet mirrors as focusing components is known for example from DE 10 2008 009 600 A1. Such facet mirrors are constructed from a multiplicity of individual mirrors or mirror facets, which each can be designed to be tiltable by way of flexure bearings for the purposes of adjusting, or else for realizing specific illumination angle distributions. These mirror facets can comprise a plurality of micromirrors in turn.
Moreover, the use of mirror arrangements which comprise a multiplicity of mutually independently adjustable mirror elements in an illumination device of a microlithographic projection exposure apparatus, designed for operation at wavelengths in the VUV range, for adjusting defined illumination settings (i.e. intensity distributions in a pupil plane of the illumination device) is also known, for example from WO 2005/026843 A2.
A problem occurring in practice is that mechanical tensions are generated during the layering process (i.e. when applying a layer stack including a reflection layer system onto the mirror substrate) during the production of such a mirror arrangement, e.g. a field facet mirror of an illumination device designed for operation in EUV, which mechanical tensions can lead to a deformation of the substrate and to an impairment of the optical imaging properties accompanying this. In order to overcome this problem, it is known to form an additional layer which compensates this mechanical tension in order to minimize the overall mechanical tension within the respective mirror element.
Moreover, in practice there is a need during the production of mirror elements to adjust the respective refractive power of the mirror element as exactly as possible (wherein this may be a refractive power of zero, corresponding to a plane mirror element, or else a refractive power differing from zero, depending on the application). An approach known to this end during the manufacturing of the mirror element consists of designing the substrate, which, inter alia, is to be coated with the reflection layer system, in accordance with the desired “end specification” of the mirror element in terms of the geometry thereof, already before the application of the layer stack, e.g. forming aspheres, fine corrections, etc., and of subsequently carrying out the coating process (i.e. the application of the layer stack including the reflection layer system), for example by using the aforementioned tension compensation, in such a way that the form of the substrate is no longer changed during the coating.
In relation to the prior art, reference is made merely by way of example to WO 2004/029692 A2, DE 10 2009 033 511 A1, DE 10 2008 042 212 A1, U.S. Pat. No. 6,011,646 A, US 2008/0166534 A1, U.S. Pat. No. 7,056,627 B2, WO 2013/077430 A1 and DE 10 2005 044 716 A1.